The present invention relates to a method for managing back-up power sources for a guidance light (taxing guidance light), an emergency light, an information telecommunication system, and the like, especially to a method for charging and a method for determining a degree in deterioration of nickel metal-hydride storage batteries used in the back-up power sources.
Nickel metal-hydride storage batteries which use hydrogen storage alloys capable of absorbing and desorbing hydrogen in a reversible manner have been utilized as power sources for cordless appliances such as telecommunication apparatuses, computers, video recording apparatuses and the like, in place of the nickel-cadmium batteries, since they can be sealed and have a higher energy density than the nickel-cadmium storage batteries.
In recent years, there is a demand for using the nickel-metal hydride storage batteries in the back-up power sources for the guidance lights, the emergency lights, the information telecommunication systems and the like, where the nickel-cadmium storage batteries had since been used, in order to design the smaller-sized appliances and to decrease a harmful influence to the environment.
Since the back-up power sources are usually provided as substitute power sources for emergency, they are required to constantly maintain satisfactory discharge capacities. There, has however been no method for charging the nickel-metal hydride storage batteries suitable for the application of the back-up power source. Therefore, they have not been widely utilized as back-up power sources.
In the conventional back-up power sources using the nickel-cadmium storage batteries, trickle charging in which the batteries are constantly supplied with minute charging currents has been employed.
When the nickel-metal hydride storage batteries are subjected to the trickle charge, the storage capacities of the batteries decrease and the batteries can not maintain a satisfactory discharge capacity. Such decrease in battery capacities is due to the proceedings of oxidation of the hydrogen storage alloy contained in the negative electrodes, facilitated by bringing the batteries into the overcharged state during the trickle charge. That is, the oxidation of the hydrogen storage alloy causes an increase in internal resistance of the batteries due to the decrease in hydrogen storaging ability of the alloy and consumption of the electrolyte.
A suitable charging method for charging back-up power sources having nickel-metal hydride storage batteries, using an intermittent charge method has been proposed in, for instance, Japanese Unexamined Patent Publication No. Hei 9-117074 or Proceedings of the 1997 communications society conference of IEICE (Proceedings II, page 531). The intermittent charge method is a method of maintaining a battery in a nearly fully-charged state by charging the battery intermittently to compensate for the self-discharge quantity of the battery during the last idle period or the last quiescence in the charging.
According to the proposal, the charge is performed while measuring a voltage of the battery and stopped when the battery voltage reaches a maximum threshold value, and resumed when the battery voltage drops to a minimum threshold value during the idle period.
Since such charge control systems are however required to constantly monitor the battery voltage, the charge control system becomes inevitably expensive. It is difficult to utilize such control systems for guidance lights, emergency lights, information telecommunication systems and the like which have limited manufacturing and operating costs.
In addition, when the internal resistance of the battery rises, for example, and the battery is near the end of its life cycle, the battery voltage does not drop to the predetermined value and the charge is not resumed even if the self-discharge quantity of the battery reaches the predetermined value.
With back-up power sources for computers and information telecommunication systems, a battery management unit (BMU) is usually used. The BMU has the function of determining the remaining servicable period of the battery by detecting the remaining discharge capacity of the battery, and also has the function of determining the remaining life period of the battery by detecting the deterioration of the battery. The BMU is expensive due to its structure. Therefore, the BMU can be applied to the back-up power sources for the computers, but can hardly be applied to guidance lights, emergency lights, information telecommunication systems and the like, where manufacturing and maintaining costs are high.
In the above-mentioned prior art, there is disclosed a method for detecting the value of the voltage drop after the idle period, i.e., the drop of the battery voltage during a predetermined period after the stop of the charging, for determining the degree in deterioration of the battery. According to this method, when the charging is performed at a small current, the voltage drop value itself is small because the voltage is also small. It is therefore difficult to detect the voltage of the battery and to determine the degree in deterioration thereof with a high accuracy.
The primary object of the present invention is to solve the above-mentioned problems and to provide a method for managing the back-up power source for nickel metal-hydride storage batteries which is capable of maintaining the discharge capacity of the batteries for a long time and is suitable for back-up power sources for guidance lights, the emergency lights, the information telecommunication systems and the like.